1. Field of the Invention
The present invention relates to a production apparatus for a gallium nitride semiconductor film, a cleaning apparatus for exhaust gas and an overall production plant for the foregoing semiconductor. More particularly, it is concerned with a production apparatus for a gallium nitride semiconductor film, a cleaning apparatus for exhaust gas and an overall production plant for the foregoing semiconductor film that are capable of preventing electrostatic charging due to ammonium chloride powders in the case of producing the gallium nitride semiconductor film.
2. Description of the Related Arts
There has been a rapid increase in recent years, in demands for a gallium nitride semiconductor film, which is a compound semiconductor as a light emitting device and photoreception device concentratedly in the field of optical communication. As a process for producing a gallium nitride semiconductor film, there are well known MOCVD (metal organic chemical vapor deposition) method which comprises reacting an organometal gas typified by trimethyl gallium as a gallium source with ammonia so that the gallium source undergoes vapor phase deposition to form a film; and HVPE (hydride vapor phase epitaxy) method which comprises reacting gallium chloride (GaCl) gas which is generated by circulating hydrogen chloride gas through metallic gallium as a gallium source, with ammonia so that the gallium source is subjected to vapor phase deposition to form a film.
In more detail, the above-mentioned HVPE method comprises synthesizing gallium chloride (GaCl) by the reaction between metallic gallium and hydrogen chloride at a temperature in the range of 800 to 1000° C., and reacting the gallium chloride thus obtained with ammonia at a temperature in the range of 800 to 1200° C. so as to subject a gallium nitride film to vapor phase deposition onto a substrate which has been arranged in advance. The vapor phase deposition is carried out usually at atmospheric pressure by means of a reaction furnace of hot wall type. Since highly corrosive hydrogen chloride is introduced in the reaction furnace at a temperature as high as 1000° C., there are generally employed as a material of construction therefor, such ceramics as quartz and the like which have excellent stability even at a high temperature and corrosion resistance.
As compared with the MOCVD method, the aforesaid HVPE method is low in the amount of ammonia gas to be used, but exhausts hydrogen gas, a slight amount of hydrogen chloride gas and a large amount of ammonium chloride powders along with a large amount of unreacted ammonia gas. Since ammonia has high toxicity as prescribed by ACGIH at a permissible concentration of 25 ppm, ammonia-containing exhaust gas which is exhausted from the reaction furnace needs to be cleaned prior to its discharge into the atmosphere.
As a method for cleaning ammonia-containing exhaust gas in a production process of a gallium nitride semiconductor film by means of the aforesaid HVPE, there are available wet absorptive method in which ammonia is neutralized by being brought into contact with an acidic aqueous solution; combustion treatment method in which ammonia is burnt by being introduced in the flame of a fuel such as propane; thermally decomposing treatment method in which ammonia is decomposed into nitrogen and hydrogen by being brought into contact with an ammonia decomposition catalyst under heating; and dry reaction method in which ammonia is cleaned through chemical reaction by being brought into contact with chemicals having chemical reactivity with ammonia; and the like method.
However, the wet absorptive method has caused a concern in that since semiconductor production equipment of atmospheric pressure system is employed in HVPE method, back diffusion of water from an acidic aqueous solution exerts adverse influence on a product.
The combustion treatment method has suffered from the disadvantage that since ammonium chloride powders bring about a fear of clogging a combustion nozzle, the powders must be disposed of in advance by using a filter or the like, where the disposal is made difficult and besides, the method has involved a fear of causing environmental pollution, since the half amount of ammonia to be treated turns into NOx.
The thermally decomposing treatment method has suffered from the disadvantage that since the amount of ammonia gas contained in exhaust gas in HVPE method is about one tenth that in MOCVD method, the production cost of an exhaust gas cleaning apparatus is relatively high, considering a small amount thereof, thus necessitating an unreasonably large installation space.
The dry reaction method has involved the problems in that since ammonium chloride powders are accumulated in the space of chemicals, thereby gradually increasing the pressure loss therethrough, the use of the chemicals is made impossible by such pressure loss before exhaustive use of the capacity of the chemicals, and also in that relatively expensive chemicals leads to a high running coat of equipment.
Under such circumstances, research and investigation have previously been made by the present inventors in order to develop an excellent cleaning apparatus free from the aforesaid shortcomings. As a result, a wet absorptive cleaning apparatus has recently been developed which is capable of preventing back diffusion of water from an acidic aqueous solution. The cleaning apparatus is that which is capable of completely preventing back diffusion of water by specifying the relationship among the cross sectional area of exhaust gas piping which connects semiconductor production equipment and a wet absorptive cleaning apparatus for exhaust gas, the length of the piping and the flow rate of the gas therein {refer to Japanese Patent Application Laid-Open No. 288342/2000}. Since an acidic aqueous solution such as hydrochloric acid is used in such a cleaning apparatus for the purpose of cleaning ammonia-containing exhaust gas, use is made of a synthetic resin having excellent corrosion resistance as a material of construction.